The first space shuttle mission since the 2003 Columbia disaster will go ahead after fixing damage to a heat-resistant tile, the US space agency says.

The tile was damaged when a window covering fell off Discovery while the spacecraft sat on the launchpad at Cape Canaveral in Florida.

The cover, used to keep windows clean until lift-off, fell about 20 metres onto one of the shuttle's orbital manoeuvring systems.

It happened as the clock was counting down to the scheduled launch at 05:51 AEST on Thursday morning (19:51 UTC on Wednesday).

The damage caused by falling debris rang alarm bells because that was precisely the problem that doomed Columbia.

It also came just hours after NASA administrator, Michael Griffin, said all issues except possible bad weather had been settled and Discovery was ready for launch.

"Everything is at rest today. Yesterday we were working a couple of ... issues and those were amply put to bed, so we're in good shape," Griffin said, adding that he hoped "the weather gods are kind for tomorrow".

"Can there be something that we don't know about that can bite us? Yeah, this is a tough business, it's a very tough business but everything that we know about has been covered."

NASA has not flown a shuttle mission since Columbia disintegrated over Texas in February 2003.

Its wing had been breached on lift-off 16 days earlier by falling foam and superheated gases rushed into the gap as the shuttle re-entered Earth's atmosphere. All seven crew died.

Discovery's mission will test improvements made to the shuttle to reduce falling debris at lift-off and experimental procedures for repairing damaged heat resistant tiles.

The window covering that fell off would have been removed before launch.

The shuttle, under the command of veteran astronaut Eileen Collins, will also deliver much-needed supplies and equipment to the International Space Station.

The station's construction, a 16-country project, has been on hold since the remaining three-shuttle fleet was grounded.

NASA weather forecasters say the outlook for launch is good, but say thunderstorms are increasingly likely.

"For our launch forecast, we did get a little more pessimistic on this today," weather officer Kathy Winters says as the countdown clock ticked toward the scheduled lift-off.

"There's a 40% chance of weather prohibiting launch," she says.

Any thunderstorm must be at least 37 kilometres from the shuttle to allow a launch. A network of 112 cameras set up to monitor Discovery's surface as it soars will need clear skies for good images.

If Discovery's launch is delayed, NASA can attempt it twice more before having to break for a few days to refuel the craft's onboard power generators.

The current launch window runs to 31 July. The next one opens on 9 September.